Plant illumination device and method

ABSTRACT

An improved method to produce artificial light for plant cultivation, an illumination device with a semiconductor light emission solution and device suited for plant cultivation in a greenhouse environment are described. The best mode is considered to be a lighting device with binary alloy quantum dots ( 110, 120, 130, 140, 150, 160 ) made by colloidal methods to produce a size distribution of quantum dots that produces an emission spectrum similar to the photosynthetically active radiation (PAR) spectrum. The methods and arrangements allow more precise spectral tuning of the emission spectrum for lights used in plant ( 310, 311 ) cultivation. Therefore unexpected improvements in the photomorphogenetic control of plant growth, and further improvements in plant production are realized.

TECHNICAL FIELD OF INVENTION

The invention relates to an improved method to produce artificial lightfor plant cultivation. In more particular, the invention relates to anillumination device with a semiconductor light emission solution suitedfor plant cultivation in a greenhouse environment.

BACKGROUND

Only approximately 50% of the radiation reaching the surface isphotosynthetically active radiation (PAR). PAR is construed to comprisethe wavelength region between 300 nm and 800 nm of the electromagneticspectrum. Photosynthesis together with photoperiodism, phototropism andphotomorphogenesis are the four representative processes related tointeraction between radiation and plants. The following expression showsthe simplified chemical equation of photosynthesis:6H₂O+6CO₂(+photon energy)→C₆H₁₂O₆+6O₂

The typical absorption spectra of the most common photosynthetic andphoto-morphogenetic photoreceptors, such as chlorophyll a, chlorophyll band betacarotene, and the two interconvertable forms of phytochromes(Pfr and Pr) are presented in FIG. 1A.

The photomorphogenetic responses, contrary to photosynthesis, can beachieved with extremely low light quantities. The different types ofphotosynthetic and photo-morphogenetic photoreceptors can be grouped inat least three known photosystems: photosynthetic, phytochrome andcryptochrome or blue/UV-A (ultraviolet-A).

In the photosynthetic photosystem, the existing pigments arechlorophylls and carotenoids. Chlorophylls are located in thechloroplasts' thylakoids located in the leaf mesophyll cells of plants.The quantity or the energy of the radiation is the most significantaspect, since the activity of those pigments is closely related to thelight harvest. The two most important absorption peaks of chlorophyllare located in the red and blue regions from 625 to 675 nm and from 425to 475 nm, respectively. Additionally, there are also other localizedpeaks at near-UV (300-400 nm) and in the far-red region (700-800 nm).Carotenoids such as xanthophylls and carotenes are located in thechromoplast plastid organelles on plant cells and absorb mainly in theblue region.

The phytochrome photosystem includes the two interconvertable forms ofphytochromes, Pr and Pfr, which have their sensitivity peaks in the redat 660 nm and in the far-red at 730 nm, respectively. Photomorphogeneticresponses mediated by phytochromes are usually related to the sensing ofthe light quality through the red (R) to far-red (FR) ratio (R/FR). Theimportance of phytochromes can be evaluated by the differentphysiological responses where they are involved, such as leaf expansion,neighbour perception, shade avoidance, stem elongation, seed germinationand flowering induction. Although shade-avoidance response is usuallycontrolled by phytochromes through the sensing of R/FR ratio, theblue-light and PAR level is also involved in the related adaptivemorphological responses.

Blue- and UV-A (ultraviolet A)-sensitive photoreceptors are found in thecryptochrome photosystem. Blue light absorbing pigments include bothcryptochrome and phototropins. They are involved in several differenttasks, such as monitoring the quality, quantity, direction andperiodicity of the light. The different groups of blue- andUV-A-sensitive photoreceptors mediate important morphological responsessuch as endogenous rhythms, organ orientation, stem elongation andstomatal opening, germination, leaf expansion, root growth andphototropism. Phototropins regulate the pigment content and thepositioning of photosynthetic organs and organelles in order to optimizethe light harvest and photoinhibition. As with exposure to continuousfar-red radiation, blue light also promotes flowering through themediation of cryptochromes photoreceptors. Moreover,blue-light-sensitive photoreceptors (e.g. flavins and carotenoids) arealso sensitive to the near-ultraviolet radiation, where a localizedsensitivity peak can be found at around 370 nm.

Cryptochromes are not only common to all plant species. Cryptochromesmediate a variety of light responses, including the entrainment of thecircadian rhythms in flowering plants such as the Arabidopsis. Althoughradiation of wavelengths below 300 nm can be highly harmful to thechemical bonds of molecules and to DNA structure, plants absorbradiation in this region also. The quality of radiation within the PARregion may be important to reduce the destructive effects of UVradiation. These photoreceptors are the most investigated and thereforetheir role in control of photosynthesis and growth is known reasonablywell. However, there is evidence of the existence of otherphotoreceptors, the activity of which may have an important role inmediating important physiological responses in plants. Additionally, theinteraction and the nature of interdependence between certain groups ofreceptors are not well understood.

Many plants can be grown in a different geographical location to theirnatural habitat by means of greenhouse cultivation utilising artificiallight. It is known from WO 2010/053341 A1 by Zukauskas et al. that lightemitting diodes (LEDs) can be used with phosphor conversion to satisfysome of the photomorphogenetic needs of plants. Phosphor conversionoperates so that there is a light such as an LED that emits at a shortwavelength adjacent to a phosphor component that absorbs and re-emitsthe radiation at a longer wavelength. This way the aggregate emissionspectrum of the lighting device can be tuned, so that the photonsprovided to the plant allow the plant to grow in a certain way, e.g. tomeet some morphological objectives such as stem height. This document iscited here as reference.

Light emitting diodes (LEDs) are increasing in popularity every day. Apeculiar new structure used for LEDs is the quantum dot that is asemiconductor whose excitons are confined in all three spatialdimensions. Quantum dots have been suggested to be used to get rid ofphosphor in WO 2009/048425 that discusses a multiple quantum wellstructure (MQW) comprising quantum dots. According to this publication,the MQW structure can be used to produce a phosphor free red and whitenitride based LED. This document is also cited here as reference.

The prior art has considerable disadvantages. The prior art fluorescencetubes, LEDs and phosphor arrangements do not allow sufficiently highresolution tuning of the emission spectra. Furthermore the prior artfluorescence tubes, LEDs and phosphor arrangements are very poor as theprimary source of light for the plants, yielding poor quality harvestsin dark growth cavities, such as basements of buildings etc.

The prior art MQW and quantum dot illumination devices are mainlyfocused on replacement of disadvantageous architectural features (suchas phosphorus), which is of little help to a horticulturalist.

Quite clearly more sophisticated plant cultivation technologies areneeded to combat global hunger in the developing countries, as well asto reduce the environmental impact of food and plant production in thedeveloped world.

SUMMARY

The invention under study is directed towards a system and a method foreffectively realising a quantum confined semiconductor illuminationdevice that addresses the photomorphogenetic needs of plants with betterprecision than ever before.

In one aspect of the invention the quantum confinement is realised as aquantum dot, i.e. confinement in all 3-spatial dimensions, or indeed asa plurality of quantum dots. Besides using quantum dots, quantum wires(2-D spatial confinement) and quantum wells (1-D spatial confinement)can be used to implement the invention in some embodiments, for exampleby replacing one or more quantum dots from said embodiments.

According to one aspect of the invention, a quantum dot—light emittingdiode features quantum dots of different sizes. In quantum dots the sizeinversely correlates with the emission energy, i.e. smaller quantum dotsemit higher energies. In one aspect of the invention the sizedistribution of quantum dots is selected so that it produces anaggregate emission spectrum with favourable photomorphogenetic effectsfor the plants that are being cultivated with the artificial lightemitted by said quantum dot—light emitting diode of the invention.

It is an aim of the present invention to eliminate at least a part ofthe problems relating to the art and to provide a new way offacilitating plant growth using quantum dots.

It is a first objective of the invention to provide a single lightemission source based quantum dot device to which the photosynthesisprocess responds well.

It is a second objective of the invention to provide a lighting fixturefor greenhouse cultivation based on a photosynthesis photon flux (PPF)optimized quantum dot device.

It is a third objective of the invention to achieve a quantum dot devicethat provides at least two emission peaks in the wavelength range from300 to 800 nm and at least one of the emission peaks has Full width athalf maximum (FWHM) of at least 50 nm or more.

It is a fourth objective of the invention to provide a quantum dot basedgreenhouse cultivation lighting fixture wherein the emission intensityratio of two emission frequencies, 300-500 nm and 600-800 nm, arereduced with less than 20% during the 10,000 hours of operation.

It is a fifth objective of the invention to provide a technical solutiongiving a better PPF value per Watt (i.e. PPFs against used powerwattage) than attained by a conventional high pressure sodium or LEDlamp normally used in greenhouse cultivation and thus providing anenergy efficient light source for greenhouse cultivation process andartificial lighting used therein.

It is a sixth objective of the invention to provide a single lightemission source wherein the emission at a frequency of 300-500 nm isgenerated by the semiconductor quantum dot chip and the emission at afrequency of 600-800 nm is generated using another quantum dot chip. Theinventor has discovered that for example cucumber and lettuce plantsreach greater length and/or mass when illuminated with the inventivehorticultural light that includes far red light (700-800 nm).

It is a seventh objective of the invention to provide a single lightemission source where the emission at frequency of 300-500 nm isgenerated by the semiconductor quantum dot chip and the emission atfrequency of 600-800 nm is generated using a second quantum dot chip,which is either driven by electric current for light emission, oroperates as an wavelength up-converter of the earlier quantum dot. Thewavelength up-conversion to produce 600-800 nm radiation is achieved byusing one or more wavelength up-conversion quantum dots in proximity ofthe first quantum dot emission source.

In this application “up-conversion” is construed as changing thewavelength of incoming absorbed light to emitted light of longerwavelengths.

It is an eighth objective of the invention to provide 400-500 nm,600-800 nm or both frequency ranges partial or complete wavelengthup-conversion of semiconductor quantum dot chip radiation, the chiphaving emission at 300-500 nm range emission range. The wavelengthup-conversion is realized by using either organic, inorganic orcombination of both types of materials.

It is a ninth objective of the invention to provide the wavelengthup-conversion using nano-sized particle material for the up-conversion.

It is a tenth objective of the invention to provide the wavelengthup-conversion using molecular like material for the up-conversion.

It is an eleventh objective of the invention to provide the wavelengthup-conversion using a polymeric material wherein the up-conversionmaterial is covalently bonded to the polymer matrix providing thewavelength up-conversion.

It is a twelfth objective of the invention to present a quantum dotbased lighting fixture where the spectral band 500-600 nm is suppressed.In this suppressed band there is hardly any or no emission at all, or inany case less emission than in either of the adjacent bands 400-500 nm,600-700 nm. The suppression can be achieved in accordance with theinvention by not having any or only a small amount of primary emissionin the band 400-500 nm, and by making sure that any up-conversion causesa wavelength shift that shifts the wavelength beyond 600 nm. It isgenerally known that green plants cannot utilize green light (500-600nm) radiation as well as the radiation in the adjacent bands, as thisradiation merely reflects from the plant rather than is being absorbedfor photosynthetic conversion.

It is a thirteenth objective of the invention to present a quantum dotbased lighting fixture that maximizes anabolic growth of plants byproviding desired far-red light, whereas it minimizes green light whichfrom the perspective of plant cultivation is radiation that wastesenergy. This objective is realized in one aspect of the invention by ablue quantum dot light emitter with a wavelength up-conversion devicewhich up-converts part of the emitted blue light (300-500) nm into abroad red spectrum component (600-800 nm) which has a far-red component,but omits and/or minimizes the green component (500-600 nm).

The present invention provides a quantum dot and a related light fixturesuitable for greenhouse cultivation. According to the invention, thequantum dot has a specific emission frequency pattern, viz. it has atleast two spectral characteristics; one emission peak with a full widthat half maximum of at least 50 nm or more and having a peak wavelengthin the range of 600 to 700 nm, and a second spectral characteristicshaving a peak wavelength below 500 nm range. The emission peaks of thequantum dots match well with a plant photosynthesis response spectrumand is therefore particularly suitable for high efficiency artificiallighting.

Some or all of the aforementioned advantages of the invention areaccrued with a quantum dot size distribution that optimises the emissionspectrum for the said photomorphogenetic variable affected, which can beany of the following biological parameters: weight, leaf number, rootmass, stem height, chemical composition (such as vitamin, mineral,and/or nutrient content and/or concentration) the plant has at differenttime points or at harvesting maturity.

A lighting device for plant cultivation is in accordance with theinvention and characterised in that said lighting device comprises aplurality of quantum dots of different size.

A lighting method for plant cultivation is in accordance with theinvention and characterised in that light is produced by a plurality ofquantum dots of different size and said light illuminates at least oneplant.

A greenhouse and/or growth chamber light device is in accordance withthe invention and characterised in that said light device comprises atleast one quantum dot.

A horticultural lighting fixture in accordance with the inventioncomprises at least one quantum dot having

a) first spectral characteristics including a peak in the wavelengthrange from 600 to 700 nm and arranged to exhibit a full width at halfmaximum of at least 50 nm or more;

b) second spectral characteristics with a maximum of 50 nm full width athalf maximum and arranged to exhibit a peak wavelength in the range from440 to 500 nm, and optionally

c) all or part of the emission at a frequency of 600-800 nm is generatedusing a whole or partial wavelength up-conversion of the quantum dotchip radiation power and/or by another electrically powered quantum dot.

A horticultural lighting fixture in accordance with the inventioncomprises at least one quantum dot having

a) first spectral characteristics including a peak in the wavelengthrange from 600 to 700 nm and arranged to exhibit a full width at halfmaximum of at least 50 nm or more;

b) second spectral characteristics with a maximum of 50 nm full width athalf maximum and arranged to exhibit a peak wavelength in the range from440 to 500 nm, and

c) at least a part or the whole of the emission at wavelengths of500-600 nm is arranged to be minimized and/or omitted and/or to bereduced below the intensity in 400-500 nm band and below the intensityin 600-700 nm band.

Use of the lighting device or fixture of any of the five precedingparagraphs is in accordance with the invention in providing light for atleast one plant with the said at least one plant in ambient light or ina dark cavity with said lighting device or fixture as the sole source oflight. Similarly a method for enhancing plant growth of the fivepreceding paragraphs is in accordance with the invention wherein atleast one lighting device or fixture emits light to at least one plantwith the said at least one plant in ambient light or in a dark cavitywith said lighting device or fixture as the sole source of light.

A light emitting component of a horticultural light is in accordancewith the invention and comprises;

-   -   a light emitting quantum dot semiconductor chip; and    -   a light wavelength up-conversion quantum dot which is deposited        in direct proximity of the quantum dot chip;        said component being capable of emitting two characteristic        light emission peaks, and at least a part or the whole of the        emission at wavelengths of 500-600 nm is arranged to be        minimized and/or omitted and/or to be reduced below the        intensity in 400-500 nm band and below the intensity in 600-700        nm band.

Use of the light emitting component of the preceding paragraph, is inaccordance with the invention, in providing light for at least one plantwith the said at least one plant in ambient light or in a dark cavitywith said lighting device or fixture as the sole source of light.Similarly a method for enhancing plant growth, is in accordance with theinvention, wherein at least one light emitting component of thepreceding paragraph emits light to at least one plant with the said atleast one plant in ambient light or in a dark cavity with said lightingdevice or fixture as the sole source of light.

A horticultural lighting fixture in a dark or shaded cavity is inaccordance with the invention and comprises at least one LED having

a) first spectral characteristics including a peak in the wavelengthrange from 600 to 700 nm and arranged to exhibit a full width at halfmaximum of at least 50 nm or more;

b) second spectral characteristics with a maximum of 50 nm full width athalf maximum and arranged to exhibit a peak wavelength in the range from440 to 500 nm, and

c) all or part of the emission at a frequency of 600-800 nm is generatedusing a whole or partial wavelength up-conversion of the LED chipradiation power.

A horticultural lighting fixture in a dark or shaded cavity is inaccordance with the invention and comprises at least one LED having

a) first spectral characteristics including a peak in the wavelengthrange from 600 to 700 nm and arranged to exhibit a full width at halfmaximum of at least 50 nm or more;

b) second spectral characteristics with a maximum of 50 nm full width athalf maximum and arranged to exhibit a peak wavelength in the range from440 to 500 nm, and

c) at least a part or the whole of the emission at wavelengths of500-600 nm is arranged to be minimized and/or omitted and/or to bereduced below the intensity in 400-500 nm band and below the intensityin 600-700 nm band.

The quantum dot and/or LED based implementations of the invention allowvery fine spectral tuning of the emission spectrum, and therefore verygood energy efficiency and improved photomorphogenetic control in plantcultivation relying on artificial light. This advantage is even morepronounced when using quantum dots only, as the spectral tuning providedby them is superior to conventional LEDs. Furthermore, the quality ofthe harvests is considerably improved with the light devices of theinvention and this brings a multitude of advantages related tocultivation in dark growth chambers or chambers with very limitedambient light: Firstly plants may be grown closer to the site ofconsumption, e.g. in residential basements in big cities, therebyeliminating transportation costs. Secondly, plants may be grown ingeographies where agriculture is not traditionally possible, e.g. hotdesert conditions in the summer.

Thirdly, as the quality of the plants is improved also the consistencybetween individual plants is improved which makes harvesting easier.This is because there are less rejected individuals and machine visionbased harvesting equipment can recognize the plants better when theyhave a consistent quality, size and colour. Fourthly, the properties ofthe plants may be varied in a controlled fashion because nearly allgrowth parameters are under control, which is especially advantageouswhen cultivating flowers and ornamental plants. Fifthly, a constantphoton dose everyday for the plants assists in the administration ofnutrients, as the nutrient dose can be maintained the same year round.Sixthly, in very hot and sunny geographies plants may be grown in darkopaque growth chambers that reflect sunlight. The energy spent in theartificial illumination of the invention is considerably less than whatwould have been expended in air conditioning or cooling the plant undersunlight.

It should be noted that a dark cavity is construed as a lightconstrained space that has zero or low levels of sunlight and/or ambientlight without the artificial light source of the invention emittingphotons, but the said cavity can be of any size, microscopically small,a flower pot size, a 10 m² residential/business basement, a shippingcargo container, the size or a football field, e.g. basement of afootball stadium, and/or a skyscraper with 20 floors where enoughvegetables are grown for an entire city at one or more floors.

In addition and with reference to the aforementioned advantage accruingembodiments, the best mode of the invention is considered to be alighting device with binary alloy quantum dots made by colloidal methodsto produce a size distribution of quantum dots that produces an emissionspectrum otherwise similar to photosynthetically active radiation (PAR)spectrum except that the emission spectrum omits or provides a very lowintensity in the green yellow (500-600) nm and comprises a highintensity spectral feature in the far red 700-800 nm band.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described in greater detail withreference to exemplary embodiments in accordance with the accompanyingdrawings, in which

FIG. 1A shows relative absorption spectra of the most commonphotosynthetic and photomorphogenetic photoreceptors in green plants.

FIG. 1B demonstrates an embodiment of the inventive lighting device 10as a block diagram.

FIG. 2 demonstrates an embodiment 20 of the lighting method inaccordance with the invention as a flow diagram.

FIG. 3 demonstrates an embodiment 30 of the use of the inventivelighting device as a block diagram.

FIG. 4 shows the embodiment 40 with emission peaks of a first singlelight emission source quantum dot device according to the invention.

FIG. 5 shows the embodiment 50 with the emission peaks of a secondsingle light emission source quantum dot device according to theinvention.

FIG. 6 shows the embodiment 60 with the emission peaks of a third singlelight emission source quantum dot device according to the invention.

FIG. 7 shows the embodiment 70 with the emission peaks of a fourthsingle light emission source quantum dot device according to theinvention.

FIG. 8 shows the embodiment 80 with the spectrum that has beendiscovered to maximize the biomass of plants according to the invention.

Some of the embodiments are described in the dependent claims.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1B shows a lighting device 100 comprising a plurality of quantumdots 110, 120, 130, 140, 150 and 160 of different sizes. The sizedistribution of quantum dots comprises quantum dots of different sizeswithin the range of 2 nm-200 nm, i.e. quantum dot 110 typically having adiameter of 200 nm and quantum dot 160 having a diameter ofapproximately 2 nm.

Electric power and electrodes (not shown) is used to produce an electricfield to excite an electron in a quantum dot in the usual way. As theelectron relaxes to a lower energy state, it emits a photon with awavelength dictated by the energy difference between the excited and therelaxed state. These emitted photons produce the emission spectrum ofthe lighting device 100.

In some embodiments the quantum dots 150, 160 are arranged to transmitUV/blue light in the 250-400 nm range, the quantum dots 140 and 130 arearranged to transmit green and/or yellow light 400-600 nm, and quantumdot 120 is arranged to transmit red light 600-700 nm, and the quantumdot 110 is arranged to transmit far red light in the band of 700-800 nm.

The relative emission intensity and number of quantum dots 110, 120,130, 140, 150 and 160 of certain size is varied to produce an aggregateemission spectrum similar and/or identical to photosynthetically activeradiation (PAR) spectrum in some embodiments. More preferably the saidemission spectrum resembling PAR spectrum omits or provides a very lowintensity in the green yellow (500-600) nm and comprises a highintensity spectral feature in the far red 700-800 nm band in accordancewith the invention.

All or some of the quantum dots 110, 120, 130, 140, 150 and 160 aretypically manufactured from any of the following alloys: cadmiumselenide, cadmium sulphide, indium arsenide, indium phosphide and/orcadmium selenide sulphide in some embodiments.

It should be noted that in more elaborate embodiments the size of atleast one said quantum dot 110, 120, 130, 140, 150 and/or 160 is chosenso that the said quantum dot produces photon emission in a band in thephoton spectrum with a certain photomorphogenetic effect in plants. Thesaid photomorphogenetic variable effected could be any of the followingbiological parameters: weight, leaf number, root mass, stem height,chemical composition (such as vitamin, mineral, and/or nutrient contentand/or concentration) the plant has at different time points or atharvesting maturity in some embodiments of the invention.

In some embodiments at least one said quantum dot 110, 120, 130, 140,150 and/or 160 is produced by colloidal synthesis. In colloidalsynthesis colloidal semiconductor nanocrystals are synthesized fromprecursor compounds dissolved in solutions, much like traditionalchemical processes. Typically the synthesis of colloidal quantum dots isbased on a three-component system composed of: precursors, organicsurfactants, and solvents. The reaction medium is heated to asufficiently high temperature, and the precursors chemically transforminto monomers. Once the monomers reach a high enough supersaturationlevel, the nanocrystal growth starts with a nucleation process. Thetemperature during the growth process is one of the critical factors indetermining optimal conditions for the nanocrystal growth in someembodiments. The temperature is typically high enough to allow forrearrangement and annealing of atoms during the synthesis process whilebeing low enough to promote crystal growth. Another critical factor thatis controlled during nanocrystal growth is the monomer concentration insome embodiments.

The growth process of nanocrystals can occur in two different regimes,usually described as “focusing” and “defocusing”. At high monomerconcentrations, the critical size (the size where nanocrystals neithergrow nor shrink) is relatively small, resulting in growth of nearly allparticles. In this regime, smaller particles grow faster than largeones, since larger crystals need more atoms to grow than small crystals,and this results in “focusing” of the size distribution to yield nearlymonodisperse particles. The size focusing is typically optimal when themonomer concentration is kept such that the average nanocrystal sizepresent is always slightly larger than the critical size. When themonomer concentration is depleted during growth, the critical sizebecomes larger than the average size present, and the distribution“defocuses” as a result of Ostwald ripening.

There are colloidal methods to produce many different semiconductors.Typical dots of the invention are made of binary alloys such as cadmiumselenide, cadmium sulfide, indium arsenide, and indium phosphide.Although, dots may also be made from ternary alloys such as cadmiumselenide sulfide in some embodiments. These quantum dots can contain asfew as 100 to 100,000 atoms within the quantum dot volume, with adiameter of 10 to 50 atoms. This corresponds to about 2 to 10nanometers.

It is in accordance with the invention to produce different populationsof quantum dots by different methods of colloidal synthesis, or othermethods, and then combine the said populations to yield a sizedistribution that provides the desired emission spectrum for plantcultivation.

It should be noted that the embodiment 10 can be used together withconventional LEDs in accordance with the invention. The embodiment 10 isalso suited to be used as illumination source for at least one plant ina dark growth cavity and/or chamber, or a cavity with low levels ofambient light.

It should also further be noted that the embodiment 10 can be readilypermuted and/or combined with any of the embodiments 20, 30, 31, 40, 50,60, 70 and/or 80.

FIG. 2 shows the operation of the lighting device of the invention, forexample in a greenhouse environment. In phase 200 light emission isdirected from the lighting device towards at least one plant.

In phase 210 electric power is provided to the lighting device and atleast one quantum dot in said lighting device, which produces anelectric field. The electric field excites an electron in a quantum dotto a higher energy state in phase 220.

As the electron relaxes to a lower energy state, it emits a photon witha wavelength dictated by the energy difference between the excited andthe relaxed state in phases 230 and 240. These emitted photons producethe emission spectrum that is transmitted from the lighting device.

In some embodiments UV/blue light in the 250-400 nm range, green and/oryellow light in the 400-600 nm range, red light 600-700 nm, and/or farred light in the band of 700-800 nm are emitted by quantum dots ofdifferent sizes in the method of the invention. Typically larger quantumdots emit red light of longer wavelength and smaller quantum dots bluelight of shorter wavelengths in some embodiments of the invention.

It should be noted that the embodiment 20 can be used together withconventional LEDs in accordance with the invention. The embodiment 20 isalso suited to be used as illumination method for at least one plant ina dark growth cavity, or a cavity with low levels of ambient light.

It should also further be noted that the embodiment 20 can be readilypermuted and/or combined with any of the embodiments 10, 30, 31, 40, 50,60, 70 and/or 80.

FIG. 3 shows different use configuration embodiments 30, 31 of theinventive artificial greenhouse illumination device and method. In oneembodiment 30 the plants 311 are cultivated on the floor of a greenhousewith transparent walls 301. A lighting device 322 with a plurality ofquantum dots is located in a position from where photons emitted by itcan reach as many plants 311 as possible with maximum emission flux. Insome embodiments the emission spectrum 350 of the lighting device isadjusted to complement the natural light spectrum that is the sunlightthat is transmitted through the wall 301. In some embodiments thelighting device 322 may comprise quantum dots arranged to transmit thosewavelengths that are filtered and/or attenuated by the greenhouse wallsin accordance with the invention.

In embodiment 31 the plants to be cultivated are stacked in growthchambers 360 in the greenhouse 300. In some embodiments each growthchamber has a lighting device 321. Even if the plants are stacked intransparent growth chambers, there is a greater reduction and/orattenuation of sunlight than in embodiment 30 as some of the photonsneed to transmit through more than one transparent wall. Therefore, thelighting device 321 with quantum dots typically complements the naturallight spectrum of multiple transmissions as above, or in the case of anopaque chamber provides all light radiation to plants 310. In someembodiments there are both dedicated lighting devices for growthchambers and at least one lighting device 320 shared by more than oneplant 310 in one or more growth chambers 360.

In some embodiments the quantum dots are arranged to produce an emissionspectrum that when combined with transmitted spectrum 340 is similar tophotosynthetically active radiation (PAR) spectrum. Preferably the saidproduced emission spectrum omits or provides a very low intensity in thegreen yellow (500-600) nm and comprises a high intensity spectralfeature in the far red 700-800 nm band in some embodiments. This isespecially preferred when the light device is used in dark growthchambers.

In some embodiments at least one quantum dot in the lighting device ischosen to emit in a band in the photon spectrum which band has a certainphotomorphogenetic effect in plants. The said photomorphogeneticvariable effected could be any of the following biological parameters:weight, leaf number, root mass, stem height, chemical composition (suchas vitamin, mineral, and/or nutrient content and/or concentration) ofthe plant 310, 311 at different time points or at harvesting maturity.

It should be noted that the embodiment 30 can be used together withconventional LEDs in accordance with the invention. The embodiments 30,31 are also suited to be implemented with growth chambers 360 of anylevel of opaqueness or transparency.

It should also further be noted that the embodiments 30 and 31 can bereadily permuted and/or combined with each other and/or with any of theembodiments 10, 20, 40, 50, 60, 70 and/or 80.

In FIG. 4, the semiconductor quantum dot chip emission frequency peaksat a wavelength of 457 nm with emission peak Full width at half maximum(FWHM) of 25 nm. In this case the wavelength up-conversion is done byusing two up-conversion materials. These two wavelength up-conversionmaterials have individual emission peaks at 660 nm and 604 nm. Thesematerials can be quantum dots in some embodiments. FIG. 4 shows thecombined emission peak from these two wavelength up-conversion materialspeaking at 651 nm wavelength with emission peak FWHM of 101 nm. In thiscase about 40% (calculated from the peak intensities) of thesemiconductor quantum dot chip emission, is up-converted to 651 nmemission by two individual up-conversion materials.

In some embodiments up-conversion is not used, and the longer wavelengthspectral feature is emitted by at least one quantum dot that is drivenby electric power.

It should be noted that the spectrum 40 can be used and implementedtogether with conventional LEDs. Spectrum 40 can be implemented by atleast one quantum dot and at least one LED in combination in accordancewith the invention. The spectrum 40 is especially suited to be used forilluminating at least one plant in a dark growth cavity, or a cavitywith low levels of ambient light.

It should also further be noted that the embodiment 40 can be readilypermuted and/or combined with any of the embodiments 10, 20, 30, 31, 50,60, 70 and/or 80.

In FIG. 5, the semiconductor quantum dot chip emission frequency peaksat a wavelength of 470 nm with emission peak Full width at half maximum(FWHM) of 30 nm. In this case the wavelength up-conversion is done byusing two up-conversion materials. These two wavelength up-conversionmaterials have individual emission peaks at 660 nm and 604 nm. Thesematerials can be quantum dots in some embodiments. FIG. 5 shows thecombined emission peak from these two wavelength up-conversion materialspeaking at 660 nm wavelength with emission peaks FWHM of 105 nm. In thiscase about 60% (calculated from the peak intensities) of thesemiconductor LED chip emission, is up-converted to 660 nm emission bytwo individual “up-conversion” materials.

In some embodiments up-conversion is not used, and the longer wavelengthspectral feature is emitted by at least one quantum dot that is drivenby electric power.

It should be noted that the spectrum 50 can be used and implementedtogether with conventional LEDs. Spectrum 50 can also be implemented byat least one quantum dot and at least one LED in combination inaccordance with the invention. The spectrum 50 is especially suited tobe used for illuminating at least one plant in a dark growth cavity, ora cavity with low levels of ambient light.

It should also further be noted that the embodiment 50 can be readilypermuted and/or combined with any of the embodiments 10, 20, 30, 31, 40,50, 60, 70 and/or 80.

In FIG. 6, the semiconductor LED chip emission frequency peaks at awavelength of 452 nm with emission peak Full width at half maximum(FWHM) of 25 nm (not shown in the FIG. 6). In this case the wavelengthup-conversion is done by using one up-conversion material. This materialcan be a quantum dot in some embodiments. FIG. 6 shows the emission peakfrom this up-conversion material peaking at 658 nm wavelength withemission peak FWHM of 80 nm. In this case about 100% (calculated fromthe peak intensities) of the semiconductor quantum dot chip emission, isup-converted to 658 nm emission by the up-conversion material. This canbe noticed from the FIG. 6, as there is no 452 nm emission exiting thequantum dot device.

In some embodiments up-conversion is not used, and the longer wavelengthspectral feature is emitted by at least one quantum dot that is drivenby electric power.

It should be noted that the spectrum 60 can be used and implementedtogether with conventional LEDs. Spectrum 60 can be implemented also byat least one quantum dot and at least one LED in combination inaccordance with the invention. The spectrum 60 is especially suited tobe used for illuminating at least one plant in a dark growth cavity, ora cavity with low levels of ambient light.

It should also further be noted that the embodiment 60 can be readilypermuted and/or combined with any of the embodiments 10, 20, 30, 31, 40,50, 70 and/or 80.

In FIG. 7, the semiconductor quantum dot chip emission frequency peaksat a wavelength of 452 nm wavelength with emission peak Full width athalf maximum (FWHM) of 25 nm. In this case the wavelength up-conversionis done by using one up-conversion material. This material can be aquantum dot in some embodiments.

FIG. 7 shows the emission peak from this up-conversion material peakingat 602 nm wavelength with emission peak FWHM of 78 nm. In this caseabout 95% (calculated from the peak intensities) of the semiconductorquantum dot chip emission, is up-converted to 602 nm emission by thewavelength up-conversion material.

In some embodiments up-conversion is not used, and the longer wavelengthspectral feature is emitted by at least one quantum dot that is drivenby electric power.

It should be noted that the spectrum 70 can be used and implementedtogether with conventional LEDs. Spectrum 70 can be implemented also byat least one quantum dot and at least one LED in combination inaccordance with the invention. The spectrum 70 is especially suited tobe used for illuminating at least one plant in a dark growth cavity, ora cavity with low levels of ambient light.

It should also further be noted that the embodiment 70 can be readilypermuted and/or combined with any of the embodiments 10, 20, 30, 31, 40,50, 60 and/or 80.

FIG. 8 shows an optimised spectrum 80 that maximises biomass productionin plants. The optimised spectrum is preferably produced with thelighting devices of the invention described in this application.Spectrum 80 has special advantages in growth chamber cultivation, wherethe growth chamber is a dark chamber, i.e. has zero or low levels ofsunlight and/or ambient light. The light device of the inventionproducing spectrum 80 can be placed into said chamber and maximisebiomass production in accordance with the invention. The inventor hasexperimentally discovered the biomass maximising feature of spectrum 80.

It should also further be noted that the embodiment 80 can be readilypermuted and/or combined with any of the embodiments 10, 20, 30, 31, 40,50, 60 and/or 70.

The used quantum dot materials and sizes should be selected in the waythat a desired emission spectra from the quantum dot device is achieved.

To summarize, by tuning the quantum dot species and size distribution itis possible to tune the desired emission spectra from the quantum dotdevice device and by tuning the quantum dot number it is possible totune the desired quantum dot chip emission quantity/amount for thequantum dot device.

The present invention also concerns a lighting fixture for facilitatingplant growth comprising at least one quantum dot having spectralcharacteristics including a peak in the wavelength range from 600 to 700nm.

By using this approach, the light sources can be designed to reachsuperior PPF and PPF per watt efficiency and performance and very lowpower consumption and very long operation lifetime when compared to theexisting technologies.

In some embodiments the emission at a frequency of 300-500 nm isgenerated by the semiconductor quantum dot chip and the emission atfrequency of 400-800 nm is generated using a complete or partialwavelength up-conversion of the quantum dot chip radiation power. Thepartial wavelength up-conversion can be selected to be in range of5-95%, preferably 35-65%, of the semiconductor quantum dot chipradiation. The wavelength up-conversion to produce the 400-800 nmradiation is achieved by using one or more up-conversion materials inproximity with the quantum dot emission source in some embodiments.

In this application “adjustable” peak wavelength as in the above isconstrued as a peak wavelength that can be adjusted during assembly ofthe lighting fixture at the factory, and/or also “adjustable” as in anadjustable dial in the lighting fixture for on site peak wavelengthadjustment. In addition adjusting the peak wavelengths of the quantumdots during manufacturing process of the device is also in accordancewith the invention, and “adjustable” should be construed to also includeadjustments made during the manufacturing process of the quantum dot.All aforementioned embodiments of an adjustable peak wavelength, or anyother adjustable light source or quantum dot variable are within thescope of this patent application.

In one special exemplary embodiment of the invention CdSe—ZnS(core-shell) quantum dot nano particles with average particle size of6.6 nm with approximately +/−0.5 nm particle size distribution weremixed with a two component silicone encapsulant resin. The mixing ratiowas 0.2 w-% of nano particles in the silicone resin. The resincontaining nano particles were dispensed as encapsulant into a plasticleaded chip carrier (PLCC) consisting a InGaN light emitting diode inthe PLCC cavity. The light emitting diodes was determined to haveelectroluminescent emission at 450 nm wavelength range.

The InGaN containing PLCC package with nano particles containingencapsulant material was connect to a DC voltage power source withforward voltage of 3.2V and current of 350 mA. The device opticalemission spectrum was characterized to result in two emission peaks oneat 450 nm wavelength range and the second at the 660 nm wavelengthrange. The 660 nm wavelength range emission peak's full width at halfmaximum was observed to be over approximately 60 nm. The intensityratios of the 450 nm and 660 nm peaks were 0.5:1. The aforementionedexperiment has been conducted by the applicant. It is in accordance withthe invention to produce several quantum dots as described above, someof different sizes. These quantum dots, one or many quantum dots may bedriven with electric current/voltage from a power source or the said oneor many quantum dots may be driven by optical excitation or both opticalexcitation and electric current/voltage from a power source inaccordance with the invention.

It is in accordance with the invention to include quantum dots withdifferent peak emissions in one luminaire and to control these in orderto provide a desirable spectral emission to achieve a determined growthresult or physiological response. In this way, the lighting system wouldallow a versatile control of lighting intensity and spectrum.Ultimately, the control of other abiotic parameters such as CO₂concentration, temperature, daylight availability and humidity could beintegrated within the same control system together with lighting,optimizing the crop productivity and the overall management of thegreenhouse.

The invention has been explained above with reference to theaforementioned embodiments and several commercial and industrialadvantages have been demonstrated. The methods and arrangements of theinvention allow more precise spectral tuning of the emission spectrumfor lights used in plant cultivation. The invention therefore realisesunexpected improvements in the photomorphogenetic control of plantgrowth, and further improvements in plant production. The invention alsoconsiderably improves the energy efficiency of plant cultivation relyingon artificial light. Furthermore, the quality of the harvests isconsiderably improved with the light devices of the invention and thisbrings a multitude of advantages related to cultivation in dark growthchambers or chambers with very limited ambient light: Firstly plants maybe grown closer to the site of consumption, e.g. in residentialbasements in big cities, thereby eliminating transportation costs.Secondly, plants may be grown in geographies where agriculture is nottraditionally possible, e.g. hot desert conditions in the summer.Thirdly, as the quality of the plants is improved also the consistencybetween individual plants is improved which makes harvesting easier.This is because there are less rejected individuals and machine visionbased harvesting equipment can recognize the plants better when theyhave a consistent quality, size and colour. Fourthly, the properties ofthe plants may be varied in a controlled fashion because nearly allgrowth parameters are under control, which is especially advantageouswhen cultivating flowers and ornamental plants. Fifthly, a constantphoton dose everyday for the plants assists in the administration ofnutrients, the nutrient dose can be maintained the same year round.Sixthly, in very hot and sunny geographies plants may be grown in darkopaque growth chambers that reflect sunlight, and are closed with lids.The energy spent in the artificial illumination of the invention isconsiderably less than what would have been expended in air conditioningor cooling the plant under sunlight.

The invention has been explained above with reference to theaforementioned embodiments. However, it is clear that the invention isnot only restricted to these embodiments, but comprises all possibleembodiments within the spirit and scope of the inventive thought and thefollowing patent claims.

REFERENCE

WO 2010/053341 A1, “Phosphor conversion light-emitting diode for meetingphotomorphogenetic needs of plants”, Zukauskas et al. 2010.

WO 2009/048425 A1, “Fabrication of Phosphor free red and whitenitride-based LEDs”, Soh et al. 2009.

The invention claimed is:
 1. A plant cultivation lighting device,comprising: a lighting system including a plurality of quantum dots ofdifferent sizes, each of the quantum dots being configured to producelight of a specific wavelength based on the size of the respectivequantum dot, wherein the light produced by the quantum dots isabsorbable by photomorphogenetic receptors of plants and thereby resultsin the lighting device producing an aggregate emission spectrum thatcauses a photomorphogenetic effect in the plants, and wherein the sizedistribution of said plurality of quantum dots produces the aggregateemission spectrum that is the same as the photosynthetically activeradiation (PAR) spectrum, except that the emission spectrum omits orprovides a very low intensity in the green yellow (500-600) nm band andcomprises a high intensity spectral feature in the far red 700-800 nmband.
 2. The lighting device as claimed in claim 1, further comprising aplurality of multiple quantum well structures for at least one lightemitting diode, and at least one quantum well structure comprisingquantum dot structures on a barrier layer.
 3. The lighting device asclaimed in claim 1, wherein the quantum dots emit photons to the plantsin a band or in various bands with preset relative intensities in thephoton spectrum that cause the photomorphogenetic effect in the plants.4. The lighting device as claimed in claim 3, wherein thephotomorphogenetic effect is one or more of the following biologicalparameters: weight, leaf number, root mass, stem height, and chemicalcomposition including one or more of vitamin, mineral, nutrient content,and nutrient concentration of the plant at different time points or atharvesting maturity.
 5. The lighting device as claimed in claim 1,wherein at least one of said quantum dots is made of any of thefollowing alloys: GaN, InGaN, AlGaN, GaAs, cadmium selenide, cadmiumsulphide, indium arsenide, indium phosphide and/or cadmium selenidesulphide or any combination thereof.
 6. The lighting device as claimedin claim 1, wherein the size distribution of the quantum dots comprisesquantum dots of different sizes within the range of 1 nm-20 nm.
 7. Thelighting device as claimed in claim 1, wherein at least one of saidquantum dots is produced by a metal organic chemical vapor deposition(MOCVD) process.
 8. The lighting device as claimed in claim 1, whereinat least one of said quantum dots is produced by a colloidal synthesisprocess.
 9. The lighting device as claimed in claim 1, wherein at leastone of the quantum dots emits in the far red 700-800 nm band.
 10. Alighting method for plant cultivation, comprising: illuminating lightproduced by a plurality of quantum dots of different sizes to at leastone plant, the light being absorbable by photomorphogenetic receptors ofthe at least one plant to cause a photomorphogenetic effect in the atleast one plant, wherein the size distribution of said plurality ofquantum dots produces an aggregate emission spectrum the same as thephotosynthetically active radiation (PAR) spectrum, except that theemission spectrum omits or provides a very low intensity in the greenyellow (500-600) nm band and comprises a high intensity spectral featurein the far red 700-800 nm band.
 11. The lighting method as claimed inclaim 10, wherein the quantum dots emit photons to the plants in a bandor in various bands with preset relative intensities in the photonspectrum that cause the photomorphogenetic effect in the plants.
 12. Thelighting method as claimed in claim 10, wherein the photomorphogeneticeffect is one or more of the following biological parameters: weight,leaf number, root mass, stem height, and chemical composition includingone or more of vitamin, mineral, nutrient content, and nutrientconcentration of the plant at different time points or at harvestingmaturity.
 13. The lighting method as claimed in claim 10, wherein atleast one of said quantum dots is made of any of the following alloys:GaN, InGaN, AlGaN, GaAs cadmium selenide, cadmium sulphide, indiumarsenide, indium phosphide and/or cadmium selenide sulphide or anycombination thereof.
 14. The lighting method as claimed in claim 10,wherein the size distribution of the quantum dots comprises quantum dotsof different sizes within the range of 2 nm-200 nm.
 15. The lightingmethod as claimed in claim 10, wherein at least one of said quantum dotsis produced by colloidal synthesis.
 16. The lighting method as claimedin claim 10, wherein the size of at least one of the quantum dotsresults in the quantum dot emitting in the far red 700-800 nm band. 17.A method for enhancing plant growth, comprising: emitting light, by theat least one lighting device of claim 1, to at least one plant with theat least one plant in ambient light or in a dark cavity, said lightingdevice being the sole source of light.
 18. The lighting as claimed inclaim 1, wherein said lighting device is surrounded by a shading cavitywhich is configured to be closed with a lid.